The driver may sleep under a mutex, and code path is:
tws_init_reqs [line 684: acquire the mutex]
tws_init_reqs [line 685]
bus_dmamap_create(BUS_DMA_WAITOK) [line 687] --> may sleep
The possible fix of this bug is to replace the "BUS_DMA_WAITOK" in bus_dmamap_create with "BUS_DMA_NOWAIT".
This bug is found by a static analysis tool written by myself, and it is checked by my review of the FreeBSD code.